Ill second guess him, i build robots and circuits. This is my forte.
It would be EXTREMELY unlikey that a ballast would be able to interfere with a 2.4 - 5.8 GHZ DIGITAL SIGNAL, You were not clear on the fact of, is it your wireless signal or actual gateway signal? I bet money you have it near a fridge or microwave, or your neighbours cordless phone is one the same frequency/channel as your router.
easy solution to this problem is simply to login to your router and cycle the channel it brodcasts on, until you find one without interference, I guaruntee you its an enviromental variable, put short, UHF radios on construction sites.
You cannot replicate interference of this kind using an analogue AM signal in your car, i chuckled, althought i applaude your self help mentality, nice to see people still try to fix their problems.
1. change the routers channel, it should be on 11, try 3-7. You may need to change the channel on your laptop, if its old, if not most new things "Auto-negotiate", google it.
2. ask your neighbours if they have bought a new wireless homephone or router recently.
3. Its not something dumb like a power saving feature on your TX broadcast signal?
god im still gigglin, went to my car and listened to the am radio
))) no offense, please dont be insulted.
Ill happily get into induction and gates but, it would probly go over everyones head. Im not flaming or challenging. Just offering another view
I live in australia, i assume your cable is the same as out, fibre to the exchange then copper to the door?
EDIT; reread the foil power cord solution.
I dont understand. I know what induction is, but you have wrapped your power cord in foil and its fixed your problem. I know what i would do if that was my ballast, wrip it open and install a schottky diode in rectification schematics.
Leaking that much EMI, tells me how ineffecient your ballast is... I hope your power bills are actually less then when you werent using it. Feel your control gear, is it warm or hot. If its hot this reinforces my point. Ineffecient design disapating unused energy as heat.